This invention pertains to coextruded plastic films, and particularly, to those films having a polypropylene layer adhered to a layer of a nylon-type polyamide. In such films, the adhesion between the nylon and polypropylene layers has been particularly troublesome. It has been surprisingly found that an excellent adhesive for the coextrusion of nylon and polypropylene is a blend of a graft copolymer of maleic anhydride onto a polypropylene backbone and an ungrafted polypropylene, the graft copolymer having been produced in a reaction carried out at a temperature of from about 80.degree. Celsius up to about 10.degree. Celsius below the melting point of the polypropylene, the graft copolymer containing from about 0.10% to about 2.5% maleic anhydride based on the total weight of the graft copolymer, the graft copolymer comprising between about 5% and about 30% by weight of the blend.
The graft copolymer reaction preferably took place in the presence of an organic peroxide. The polypropylene layer may contain up to about 4% polymeric ethylene.
In the coextrusion of a 5-layer film, the structure includes a fourth layer, of ethylene vinyl alcohol copolymer (EVOH), affixed on one of its surfaces to the nylon, and a fifth layer, of nylon, affixed to the other surface of the EVOH.
Thus the process comprises coextruding polypropylene, and nylon with the adhesive blend juxtaposed therebetween. In the formation of a five layer structure containing EVOH, the process comprises coextruding the five layer structure of, in sequential order in the structural layering, from one outside surface to the other: polypropylene, adhesive blend, nylon, EVOH, and nylon.